Radiotherapy is second to surgical operations as a method of locally treating malignant tumors. Because it can be applied to elderly patients and also enables normal organs and tissues to be conserved, the number of patients being treated by the method is recently increasing very rapidly. However, linear accelerator-generated high-energy X-rays and electron beams that are generally used for the radiotherapy are low linear energy transfer (LET) radiation having a relatively low biological effect. Accordingly, linear accelerator based radiotherapy has little effect on tumors such as malignant melanoma, various types of sarcoma and glioblastoma multiforme and the like. Also, because locally advanced neoplasms that have grown to several centimeters or more have many hypoxic cancer cells or contain large amounts of anti-oxidative enzyme, and are therefore resistant to radiation, linear accelerator based radiotherapy has little effect.
Heavy particle beam therapy is a method used to improve the radiotherapeutic effect on these. However, the widespread use of heavy particle beam therapy is made difficult due to the large cost of the equipment, which involves an investment of several tens of billions of yen.
Since before, various radiosensitizers (such as, for example, Metronidazole, Misonidazole, Etanidazole, Nimorazole, and so forth) have been developed to increase the radiotherapeutic effect (see non-patent documents 1 to 3, for example). However, in addition to the uncertainty of their effect, they have side effects such as peripheral neuropathy and the like, and therefore are not yet allowed to be used in clinical practice.
Using radioresistant osteosarcoma cell lines (HS-Os-1), the present inventors previously confirmed that sensitivity to the radiation effect could be increased and apoptosis readily induced by adding a low concentration of hydrogen peroxide to a culture solution during the irradiation. Normally, such radioresistant osteosarcoma cells or chondrocytes suffer little DNA oxidative damage even when exposed to radiation of 30 Gy. In contrast, when irradiated in the presence of a low concentration (0.1 mM, for example) of hydrogen peroxide, reactive oxygen species can be clearly produced in cells exposed to radiation in the order of 10 Gy, giving rise to apoptosis. One cause that can be cited for the radiation resistance of osteosarcoma cells is the presence of anti-oxidative enzyme (peroxidase, catalase) in the cells. Hydrogen peroxide is thought to block the action of the anti-oxidative enzyme. That is, when irradiation is conducted in the presence of hydrogen peroxide, the action of the anti-oxidative enzyme is blocked and oxygen is produced, oxidizing the radioresistant hypoxic cells, thereby converting them to being highly sensitive to radiation (see, for example, non-patent documents 4 to 6).
Non-patent document 1: Chapman J D, Whitmore G F (eds): Chemical modifiers of cancer treatment. Int J Radiat Oncol Biol Phys 10: 1161-1813, 1984.
Non-patent document 2: Coleman C N.: Hypoxic cell radiosensitizers: Expectations and progress in drug development. Int J Radiat Oncol Biol Phys 11: 323-329, 1985.
Non-patent document 3: Radiobiology for the Radiologist (Sixth Edition) by Eric J. Hall and Amato J. Giaccia, Lippincott Williams & Wilkins, 419-431, 2006.
Non-patent document 4: Yasuhiro Ogawa et al.: Mechanism of apoptotic resistance of human osteosarcoma cell line, HS-Os-1, against irradiation. International Journal of Molecular Medicine 12: 453-458, 2003.
Non-patent document 5: Yasuhiro Ogawa et al.: Apoptotic-resistance of human osteosarcoma cell line HS-Os-1 to irradiation is converted to apoptotic-susceptibility by hydrogen peroxide: A potent role of hydrogen peroxide as a new radiosensitizer. International Journal of Molecular Medicine 12: 845-850, 2003.
Non-patent document 6: Yasuhiro Ogawa et al.: Immunocytochemical characteristics of human osteosarcoma cell line HS-Os-1: Possible implication in apoptotic resistance against irradiation. International Journal of Molecular Medicine 14: 397-403, 2004.